Field of the Invention
The present invention relates to a group-III-nitride layer structure for a normally-off transistor. The invention further relates to a wafer comprising such a layer structure on a substrate, to a normally-off transistor, in particular a high-electron-mobility transistor (HEMT), and to an integrated circuit.
Description of the Background Art
Group-III-nitride heterojunction field effect transistors (HFETs) are widely investigated for high power switching applications. Most of the reported HFETs are AlGaN/GaN HFETs of the normally-on type, taking advantage of the inherent high sheet carrier density caused by a built-in polarization electric field.
However, such normally-on HFETs are not useful for actual power switching applications, in which safety-off operation is the main concern. Thus, in these applications, Si-based power MOSFETs/IGBTs have been exclusively used so far.
Normally-off operation, i.e., transistor operation in an off-state at a gate-source voltage of 0V or, more generally speaking, at gate-source voltages below a threshold voltage amount, is strongly desired for AlGaN/GaN HFETs. At the same time, it is desired to achieve a low on-state resistance of the transistor. In order to meet such requirements, it is necessary to reduce the two-dimensional electron gas (2DEG ) density in the channel under the gate for gate-voltages <0V. Since the 2DEG is caused by a difference of polarization-induced fixed charges between the GaN and AlGaN layers at the heterointerface, a reduction of the Al mole fraction or of the thickness of AlGaN effectively reduces those carriers and thereby shifts a threshold voltage Vth for on-state operation toward the positive direction. This approach was reported in the publication M. A. Khan, Q. Chen, C. J. Sun, J. W. Yang, M. Blasingame, M. S. Shur, and H. Park, “Enhancement and depletion mode GaN/AlGaN heterostructure field effect transistors,” Appl. Phys. Lett. vol. 68, no. 4, 514-516 (1996), hereinafter Khan et al. This approach can produce the normally-off operation. However, the resultant drain current is very low, since the applicable positive gate voltage is limited by a relatively low barrier height with high sheet resistance at the GaN/AlGaN heterointerface. Thus, the desired reduction of the on-state resistance is limited with this approach. This approach also lacks the ability to drive the gate with a high positive voltage typically. Instead, the driving voltage is typically limited to less than 2V. Khan et al. describe a device featuring a very thin AlGaN barrier layer with a threshold voltage Vth<50 mV.
Several different approaches to the design of normally-off HEMTs have been published in the past years. One structure is found in Kumar V, Kuliev A, Tanaka T, Otoki Y, Adesida I., “High transconductance enhancement mode AlGaN/GaN HEMTs on SiC substrate”, Electron Lett. 2003; 39 (24):1758-60. The authors report Vth=75 mV for a recessed gate device.
Cai et al., in Cai Y, Zhou Y, Chen K, Lau K., “High-performance enhancement-mode AlGaN/GaN HEMTs using fluoride-based plasma treatment”, IEEE Elec Dev Lett. 2005; 26 (7):435-7, rely on fluoride-based plasma treatment to achieve a threshold voltage of Vth=900 mV. While the last method can deliver a normally-off device, stability concerns as well as degradation of the device performance due to either recess etching or F ion treatment under high voltage and high temperature operations have limited this approach to R&D work only.
A more recent approach proposed by Mizutani et al. in Mizutani T, Ito M, Kishimoto S, Nakamura F., “AlGaN/GaN HEMTs with thin InGaN cap layer for normally off operation”, IEEE Elec Dev Lett. 2007; 28 (7):549-51 adopts a thin InGaN cap layer, which raises the conduction band, thereby achieving a normally-off operation. While being good at achieving positive threshold voltage, reliability of a recess etching, which is still required between Gate and Drain contacts to lower the on-state resistance, are major issues with this approach.
Due to the inherent difficulties in achieving a normally-off nitride-based HEMTs, a normally-on HEMT can also be used in a cascaded configuration with a Si FET to make a normally-off device (U.S. Pat. No. 8,084,783). This approach can realize a normally-off GaN HEMT with threshold voltage of up to 4V, but also suffers from additional chip integration and relies on a slow internal body of the Si FET.